This invention relates to propylene polymer compositions which contain an acid scavenger and more particularly relates to a propylene polymer composition containing mono potassium citrate as an acid scavenger.
Most practical uses of propylene polymer compositions require that the composition be stabilized with an acid scavenger. Typically, propylene polymers, which have been formed using catalysts containing halides, require some functionality of an acid scavenger to stabilize the polymer formulation against corrosivity for long term uses.
Many applications of propylene polymers, such as formation of spun bonded materials and molding applications including thermoforming, preferably require a resin which does not produce large amounts of smoke during processing, and more particularly, does not produce a build-up of wax-like material around the processing equipment. In many instances, this wax-like material originates from fatty acid derivatives contained in the polymer formulation. Thus it has been long known that typical acid neutralizers used in conventional polypropylene resins such a calcium stearate which contain long aliphatic chains, such as fatty acid salts, should be avoided. One alternative acid neutralizing additive used commercially, calcium lactate, does not produce as much smoke as calcium stearate, but has shown to exhibit screen pack pluggage during melt extrusion of the polymer. This limits production rates of polymer in a commercial facility and may add significant cost to production. Since calcium lactate melts above the temperature of polymer melt extrusion, its particle size is critical in filtering the propylene polymer blended with the additive. There is a need for a propylene polymer formulation which is sufficiently acid stabilized, does not produce excessive smoke or wax build-up during processing, does not cause excessive screen pack pluggage problems, and is compatible with other typically used polymer additives.
U.S. Pat. No. 4,649,171 shows polypropylene compositions containing trisodium and tripotassium citrates as anti-corrosion additives. However, due to the high melting points of these trimetal salts, these materials do not disperse well in the polymer composition and lead to filterability difficulties and high screen pack plugging.
U.S. Pat. No. 5,834,541, incorporated by reference herein, describes a polymer composition containing a non-fatty acid salt acid neutralizer in combination with specific phosphites together with certain hydroxylamines or a bezofuranone. Use of potassium citrate is not disclosed.
In one aspect of this invention, mono potassium citrate is incorporated into a propylene polymer composition to scavenge or neutralize excess acid. In other aspects of this invention, an additive package including mono potassium citrate as an acid scavenger together with other typical additive components are incorporated into a propylene polymer composition.
Further aspects of this invention include an acid stabilized propylene polymer composition which does not experience excessive screen pack plugging during the manufacturing process and which does not produce excess smoke or wax build-up around process equipment during processing.
Other aspects of the invention include formed articles produced using the compositions described in this invention and methods of producing those articles.
Still other aspects of this invention are apparent as described herein.